WO 2010/072710 discloses (FIG. 1) a power plant having a gas turbine unit 1 that comprises a compressor 2, a combustion chamber 3 and a turbine 4.
A mixture 6 comprising fresh air 7 coming from the environment is fed into the compressor 2 and flue gases 8 (deriving from the combustion of the mixture 6 with a fuel within the combustion chamber 3) emerge from the turbine 4.
These flue gases 8 (that typically have a high temperature) are fed into a boiler 9 of a steam turbine unit 10; within the boiler 9 the flue gases 8 transfer heat to water of the steam unit 10.
From the boiler 9, the flue gases 8 are supplied into a diverter 11, to be splitted into a recirculated flow 12 and a discharged flow 13.
The recirculated flow 12 is cooled in a cooler 14 and supplied via a fan 15 into a mixer 16, to be mixed with the fresh air 7 and form the mixture 6 that is fed into the compressor 2.
The discharged flow 13 is cooled in a cooler 19 and is then fed, via a fan 20, into a CO2 capture unit 21 to be then discharged into the atmosphere via 22; in contrast the CO2 that is captured in the CO2 capture unit 21 is stored in 24.
During operation, from the one side it is advantageous to have a large recirculated flow 12, since this reduces the discharged flow 13 and increases the discharged flow CO2 concentration and, therefore, it reduces the plant and operating costs (in particular with reference to the CO2 capture unit 21); from the other side it is advantageous to have low recirculated flow 12, since this increases the oxygen content in the combustion chamber 3 and improves combustion.
Therefore the amount of recirculated flow is determined by an optimisation process that balances these opposing needs.
The combustion chamber 3 of the gas turbine unit 1 is known to have a plurality of mixing devices 25 connected to a combustion device 26.
The fuel 27 is injected into the mixing devices 25 such that it mixes with the flue gases/fresh air mixture 6 to then burn.
It is clear that combustion chambers 3 (i.e. their mixing devices 25 and combustion device 26) must be designed such that at the design operating conditions (including for example recirculated flow mass flow rate, fuel composition, temperature) a design fuel only burns when it moves out of the mixing devices 25 and enters the combustion device 26, because combustion in the mixing devices 25 (so-called flashback) is very detrimental for the service life of the combustion chamber.
For this reason, when the combustion chamber 3 is designed to operate with a given fuel at given conditions, a change of the fuel may not be possible or may require the operating conditions to be changed and adapted to the features of the actual fuel being used.
Typically combustion chambers are designed for operation with a gaseous fuel (typically “standard” natural gas, i.e. natural gas of a given composition) having given features.
Nevertheless, during operation it is often necessary to switch from a fuel having design features to a different fuel having different features.
In case one of these fuels has a high or very high reactivity, it can start to burn immediately after its injection into the mixing device (i.e. before it enters the combustion device), causing flashback.
For example, natural gas is a mixture of gas containing methane (CH4), ethane (C2H6), propane (C3H8) butane (C4H10) etc and, in some cases, also H2.
The content of ethane (C2H6)+propane (C3H8)+butane (C4H10)+etc defines the C2+ (usually in mol fraction), in other words the C2+ content is the mol fraction of higher alkane species within the fuel (gaseous fuel).
When the composition of the natural gas varies (for example the amount of the C2+ and/or H2 increases when compared to the standard natural gas) its reactivity also varies and can greatly increase.
In these cases, when switching from standard natural gas to high reactive gas, the simple change of fuel would cause the new fuel to start to burn in the mixing devices 25 instead of the combustion device 26 (flashback).
To prevent this, traditionally the combustion chambers 3 are operated at a lower temperature (i.e. the flame temperature is reduced), such that the reactivity (that depends on a number of factors and also on temperature) decreases to a value allowing the fuel to correctly mix, pass through the whole mixing devices 25 and enter the combustion device 26, before it starts to burn.
In addition, also in case no fuel switch is foreseen, in some cases the features of the fuel being used may vary during operation; for example, in case natural gas is used, its C2+ and/or H2 content (and consequently its reactivity) may vary during operation.
Also in this case, in order to allow a correct operation and to prevent fuel combustion within the mixing devices (flashback), the combustion chamber is traditionally operated at a lower temperature than the design temperature, to guarantee a safety margin from flashback.
It is nonetheless clear that such an operation with reduced combustion temperature inevitably causes a loss of power and reduction of performances and efficiency.
In addition, in particular when the H2 content is large, the fuel is diluted (in some cases up to 50% or more). Such a large dilution can cause problems at the fuel supply circuit (in particular injectors and pumps), since the flow to be actually treated is much larger that the design flow.